Klez-H is the worst virus ever – official

Klez-H is the worst virus ever, according to figures from managed services firm MessageLabs, which has blocked 775,000 copies of the pathogen since it first appeared on April 15. Klez-H overtakes the infamous SirCam worm.

MessageLabs is blocking 20,000 Klez-H infected emails per day. The virus accounts for one in 300 of the emails it scans.

Although the infamous Love Bug virus is generally regarded as the fastest spreading virus ever, and accounted for one in 28 at its peak, its outbreak only lasted a short time, while Klez-H has continued to spread.

Alex Shipp, MessageLabs chief ant-virus specialist, says the reason for Klez.H's 'success' lies in its ability to cover its tracks and deceive recipients.

"Klez-H is able to select random names from address books to use as the sender address, and also creates a large range of subject, text and attachment names, making it difficult to identify and track," he said.

"These elements have meant that the virus has been able to sustain high levels of activity over many weeks and not follow the usual peak and trough pattern that occurs in most outbreak situations."

Klez-H is a variant of the first Klez virus which appeared in December 2001 and represents a fresh trend in virus writing by continually modifying viruses until they foil enough anti-virus scanners to spread successfully to email users worldwide. By contrast, Klez-I, another variant which was released by the writer on the same day as Klez-H, barely made an impact. ®